On Oct 4, 2007, at 8:24 , Brian Aker wrote:
It would be nice to do three additional things with tags:
1) Get a list of all active keys associated with a tag (even if it
is lossy). Two optimizations would be:
* Give me the count of objects within this tag
* For a read, take a snapshot version of the list to have read
consistency (aka take an MVCC approach)
Tags aren't intended to be an index, just a mass invalidation
mechanism. It's *possible* to use them as an index, but not in any
cheap way.
Without indexing, you'd have to scan all of the stored items and for
each one scan your tags and see if there is a match. Without major
changes to memcached, I don't see how you could do that without
having the server hang.
If something like that did exist, people would just start tagging
thing with ``all'' and using it for a key dump. If a key dump
couldn't exist before, I think it'd be even less likely with tags.
2) Allow the "set" of an object with its tag name. This will solve
the problem of creating an object and then tagging the object.
I don't see how there could be a problem here at all. It's almost
the same amount of information over the wire, but with two commands,
you don't clutter up existing concepts.
When we were originally covering tags (in the first binary protocol
discussion), I suggested leaving room for tags in set. Brad
disagreed and said small, orthogonal commands are much better. I
agree with his disagreement.
If you can prove that setandtag is *considerably* cheaper than set
+tag, then it could possibly be added as an extension.
3) Clear an object from a tag.
You mean untagging an object? It's not clear to me how that would
ever be used. I imagine people will be setting and tagging around
the same time, and would never want to go back and say, ``no, that
really *isn't* dependent on this information.''
How would you imagine this being used?
4) Option to invalidate a tag, and blow away all keys associated
with it (think of it as a mini-operation to save on round trips)
That's the purpose of invalidate_tag, however, it needs to execute
in constant time.
[It's unclear whether it's worth the effort to ever release a tag
once it's been added. If we assume that tags live forever, we
don't have to refcount them and a few things get easier. Any
opinions?]
If you invalidate a tag, the tag should go away.
OK, it's clear now. :)
--
Dustin Sallings
